Sort Java collection relative to the current date

Question

I want to sort my List of Date relative to the current date, e.g we have next items in list:

10.01.2018, 
10.20.2018, 
10.14.2018, 
10.02.2018 

and the current date is 10.08.2018.

The result should be ascending sort of array in the next order:

10.14.2018, 
10.20.2018    and then 
10.01.2018, 
10.02.2018. 

First should be dates that didn't happen and then the past dates. How to do it with Comparator?

Answer 1

The most concise and elegant, yet readable way I've found is as follows:

list.sort(
    Comparator
    .comparing( LocalDate.now()::isAfter )
    .thenComparing( Comparator.naturalOrder() )
);

This reads as sort by first comparing whether each date is after today or not, then break ties using LocalDate's natural order. (It's worth remembering that sorting boolean values means putting falses at the beginning and trues at the end, i.e. false < true).

Answer 2

    LocalDate now = LocalDate.now();

    List<LocalDate> dates = Arrays.asList(
            LocalDate.parse("2018-10-02"),
            LocalDate.parse("2018-10-20"),
            LocalDate.parse("2018-10-14"),
            LocalDate.parse("2018-10-01"));

    // sort relative to the current date
    dates.sort(Comparator.<LocalDate>comparingInt(localDate ->
            localDate.isAfter(now) ? -1 : localDate.isBefore(now) ? 1 : 0)
            // then sort normally
            .thenComparing(Comparator.naturalOrder()));

    System.out.println(dates);